THE PRESENT INVENTION relates to a gas generator and more particularly relates to a gas generator suitable for use in a safety arrangement intended to be incorporated into a motor vehicle such as a motor car.
It has been proposed previously to provide motor cars with safety arrangements in the form of so-called "air-bags".
Such air-bags are mounted at a convenient position within a motor vehicle and are arranged so that, in the event of an accident arising, the air-bag is inflated to form a cushion for a passenger within the motor car. A typical conventional air-bag had an inflated volume of between 60 and 160 liters.
A typical prior proposed air-bag is inflated with a gas which is generated rapidly, subsequent to an accident occurring, by a gas generator. A typical gas generator utilises a material such as a mixture of sodium um azide (NaN.sub.3), Potassium nitrate (KNO.sub.3) and Silicon dioxide (SiO.sub.2) in the form of pellets. These pellets, when ignited, produce rapidly a non-poisonous gas which consists primarily of nitrogen. 1 gram of the tablets provides 1 liter of gas and thus a total charge of between 60 and 160 grams of pellets is required.
Whilst the pellets, when ignited, produce a non-poisonous gas, the pellets also produce dust, which is poisonous. The hot dust could also damage the interior of the bag. Therefore in prior proposed air-bags which utilise the pellets as described, a filter is provided between the pellets and the interior of the bag. U.S. Pat. No. 3,904,221 discloses a typical prior art arrangement which is relatively expensive to produce and which is bulky. The pressure within an air-bag when inflated by a gas generator of this type is in the region of 0.5 bar and the time taken to inflate the bag is typically in the region of 25-30 ms.
One disadvantage of an air-bag system as described above is that of expense. Consequently it has been proposed that if a complement is required to a conventional air-bag, or to a safety-belt, in the form of an additional inflatable arrangement, then for economic reasons this complement must be cheap to produce.
It has been considered desirable to provide an inflatable bag located to one side of each passenger, the volume of which is in the region of 5-25 liters, as a complement to a conventional air-bag or safty-belt. These bags can be termed "inflatable paddings". These inflatable paddings are intended to provide protection against the side impact, but if such inflatable paddings are to be utilised they must be relatively cheap to manufacture, and thus the prior proposed gas generators are not really suitable.
It is to be understood that if an inflatable padding is to be of use in protecting a passenger against the side impact, then that inflatable padding must be inflated in an extremely short period of time subsequent to the sensing of an accident. The reason for this is that in the case of a side impact, typically the door structure of the vehicle will begin to collapse as soon as the impact occurs, since the door structure is not very strong. A sensor is provided which is responsive to a side impact. Such a sensor may respond to deformation of the door panel. However, even if the sensor responds very rapidly it is to be understood that by the time the sensor responds the door will have significantly collapsed, and the interior of the door will be extremely close to the person sitting inside the car. Thus the bag must be inflated extremely rapidly if it is to be of any real use. It is also to be appreciated that the ideal location for such an inflatable padding is within the door itself, and thus the bag needs to be extremely compact when de-flated, and the associated gas generator must be small.
It has been found that the pressure within such an inflated padding should be somewhat higher than in a conventional air-bag, preferably in the region of 1-2 bars.
It has also been proposed to use cellulose nitrate for a gas generator in an air-bag. British Patent Specification 1371506 discloses such an arrangement in which 75 grams of propellent are utilised, consisting principally of a mixture of nitro glycerine and nitro cellulose. Such a propellent will produce gas having a volume of approximately 180 liters when ignited. The gas produced is toxic and thus the arrangement disclosed in the British Patent Specification includes means provided to avoid problems that may arise if the toxic gas emerges from the bag and is breathed by people restrained by the bag. In the Specification there is a teaching that a vapourisable cooling liquid should be provided, which effects purification of the combustion gases by oxidation of the toxic constitutes thereof to form non-gaseous products. The apparatus disclosed is therefore relatively complicated.